Patent application title: Thermal Processing of Silicon Wafers

Abstract:

Apparatus and methods that minimize surface defect development in silicon
wafers during thermal processing at relatively high temperatures at which
silicon wafers are annealed and at less extreme temperature, or for other
purposes. The apparatus and methods have utility to horizontally-disposed
furnaces for silicon wafers and to vertically-oriented furnaces in which
larger wafers can be thermally processed. A selectively-sealable process
tube encloses silicon wafers during heating of the silicon wafers to a
predetermined temperature, and a heating atmosphere supply system induces
through the process tube a positive flow of a process gas, such as
hydrogen or argon, that is non-reactive with solid silicon at the
predetermined temperature. A process tube outlet vents gas from the
process tube, and an impurity sensor in the process tube outlet detects
oxygen and moisture in the vented gas to verify the purity of the
atmosphere surrounding the wafers during thermal processing.

Claims:

1. An apparatus for thermally processing silicon wafers, the apparatus
comprising:(a) a selectively-sealable process tube configured to enclose
silicon wafers during heating of the silicon wafers to a predetermined
temperature;(b) a heating atmosphere supply system communicating with the
process tube and inducing therethrough a positive flow of a process gas,
the process gas being non-reactive with solid silicon at the
predetermined temperature;(c) a process tube outlet venting gas from the
process tube; and(d) an impurity sensor in the process tube outlet, the
impurity sensor detecting each of oxygen and of moisture in gas vented
from the process tube.

2. An apparatus as recited in claim 1, wherein the process gas is chosen
from a group of gasses comprising hydrogen and argon.

3. An apparatus as recited in claim 1, wherein the heating atmosphere
supply system comprises:(a) a source of the process gas;(b) a process gas
supply line communicating between the source of the process gas and the
process tube; and(c) a point-of-use purifier interposed in the process
gas supply line to the flow of the process gas therethrough.

4. An apparatus as recited in claim 3, wherein the point-of-use purifier
comprises:(a) a filter of particulate media in the process gas; and(b) an
absorbent of metallic contaminants in the process gas.

5. An apparatus as recited in claim 1, wherein the heating atmosphere
supply system comprises:(a) a source of the process gas;(b) a process gas
supply line communicating between the source of the process gas and the
process tube;(c) a bypass loop communicating with the process gas supply
line and drawing therefrom a sample of gas flowing through the process
gas supply line to the process tube; and(d) a dew point meter in the
bypass loop, the dew point meter detecting moisture in the sample of gas
drawn through the bypass loop.

6. An apparatus as recited in claim 1, wherein the heating atmosphere
supply system comprises an argon-activated pneumatic control valve.

7. An apparatus as recited in claim 1, wherein the process tube comprises
a vertically-oriented quartz cylinder having a closed upper end and an
interior diameter of about 10 inches.

8. An apparatus as recited in claim 1, further comprising:(a) a load lock
having an open side alignable with an open end of the process tube, the
load lock being configured to enclose silicon wafers assembled for
transfer into the process tube;(b) a load lock purge system communicating
with the load lock and inducing therethrough a positive flow of a staging
gas chosen from a group of gasses comprising hydrogen and argon; and(c) a
load lock outlet venting gas from the load lock.

9. An apparatus as recited in claim 1, wherein the predetermined
temperature is in a range of from about 1175 degrees C. to about 1250
degrees C.

10. An apparatus as recited in claim 9, wherein the predetermined
temperature is about 1200 degrees C.

11. An apparatus for thermally processing silicon wafers, the apparatus
comprising:(a) a selectively-sealable vertically-oriented process tube
having a closed upper end and being configured to enclose silicon wafers
during heating of the silicon wafers to a predetermined temperature;(b) a
process tube cap capable selectively of closing the lower end of the
process tube through movement into sealing engagement with the sides of
the lower end of the process tube; and(c) a cap purge system configured
to deliver argon gas across a gap between the cap and the sides of the
lower end of the process tube, when the cap moves into sealing engagement
therewith.

12. An apparatus as recited in claim 11, wherein the cap purge system
comprises an argon-activated pneumatic control valve.

13. An apparatus as recited in claim 11, further comprising:(a) a load
lock having an open side alignable with the lower end of the process
tube, the load lock being configured to enclose silicon wafers assembled
for transfer into the process tube;(b) a load lock purge system
communicating with the load lock and inducing therethrough a positive
flow of a staging gas chosen from a group of gasses comprising hydrogen
and argon; and(c) a load lock outlet venting gas from the load lock.

14. An apparatus as recited in claim 13, further comprising an impurity
sensor in the load lock outlet, the impurity sensor detecting each of
oxygen and of moisture in gas vented from the load lock.

15. An apparatus as recited in claim 13, wherein the load lock purge
system comprises an argon-activated pneumatic control valve.

16. A method for thermally processing silicon wafers, the method
comprising the steps of:(a) assembling in a staging area a silicon wafer
to be thermally processed;(b) purging the atmosphere in the staging area
with a staging gas selected from a group of gasses comprising hydrogen
and argon;(c) transferring the silicon wafer from the staging area to a
heat treatment area;(d) surrounding the silicon wafer in the heat
treatment area with a process gas, the process gas being non-reactive
with solid silicon at a predetermined temperature; and(e) heating the
silicon wafer to the predetermined temperature in the process gas in the
heat treatment area.

17. A method as recited in claim 16, wherein the predetermined temperature
is in a range of from about 1175 degrees C. to about 1250 degrees C.

18. A method as recited in claim 17, wherein the predetermined temperature
is about 1200 degrees C.

19. A method as recited in claim 16, wherein the step of purging the
atmosphere in the staging area comprises the steps of:(a) inducing a
positive flow of the staging gas through the staging area;(b) venting gas
from the staging area during positive flow of the staging gas
therethrough;(c) monitoring gas vented from the staging area to detect
the presence therein of each of oxygen and of moisture; and(d) proceeding
with the step of transferring the silicon wafer, when oxygen and moisture
detected in gas vented from the staging area are each below respective
predetermined allowable levels.

20. A method as recited in claim 19, wherein the step of inducing a
positive flow of the staging gas comprises the step of operating an
argon-activated pneumatic control valve.

21. A method as recited in claim 16, wherein the step of surrounding the
silicon wafer comprises the steps of:(a) inducing through the heat
treatment area a positive flow of a process gas selected from a group of
gasses comprising hydrogen and argon;(b) venting gas from the heat
treatment area during positive flow of the process gas therethrough;(c)
monitoring gas vented from the heat treatment area to detect the presence
therein of each of oxygen and of moisture; and(d) continuing with the
step of heating the silicon wafer, when oxygen and moisture detected in
gas vented from the heat treatment area are each below respective
predetermined allowable levels.

22. A method as recited in claim 16, wherein the step of surrounding the
silicon wafer comprises the steps of:(a) purifying to a predetermined
quality level a process gas selected from a group of gasses comprising
hydrogen and argon, thereby producing a pure process gas;(b) removing
from the pure process gas all particulate media and all metallic
contaminants; and(c) supplying the pure process gas to the heat treatment
area after the step of removing.

23. A method as recited in claim 22, wherein the pure process gas includes
less than one part per million of each of oxygen, nitrogen, and water.

24. A method as recited in claim 22, wherein the step of supplying the
pure process gas comprises the step of operating an argon-activated
pneumatic control valve.

25. A method as recited in claim 16, wherein the step of surrounding the
silicon wafer comprises the steps of:(a) monitoring the gas entering the
heat treatment area to detect the presence of moisture therein; and(b)
continuing the step of heating the silicon wafer, when moisture detected
in the gas entering the heat treatment area is below a predetermined
allowable level.

26. A method as recited in claim 25, wherein the step of monitoring the
gas entering the heat treatment area comprises the steps of:(a) diverting
a sample of the gas entering the heat treatment area; and(b) evaluating
the sample of the gas with a dew point meter.

27. A method as recited in claim 16, wherein the step of transferring the
silicon wafer comprises the steps of:(a) removing a barrier interposed
between the staging area and an opening into the heat treatment area;(b)
moving the silicon wafer from the staging area through the opening into
the heat treatment area;(c) closing the opening into heat treatment area
with the barrier.

28. A method as recited in claim 27, wherein the step of closing the
opening into the heat treatment area comprises the steps of:(a) advancing
the barrier into sealing engagement with the sides of the opening into
the heat treatment area; and(b) delivering argon gas across a gap between
the barrier and the sides of the opening into the heat treatment area
during the step of advancing the barrier.

29. A method as recited in claim 28, wherein the step of delivering argon
gas comprises the step of operating an argon-activated pneumatic control
valve.

Description:

BACKGROUND

[0001]A. Technical Field

[0002]The present invention relates generally to the thermal processing of
silicon wafers. More particularly, the present invention pertains to the
high temperature annealing of silicon wafers.

[0003]B. Background of the Invention

[0004]The annealing of silicon wafers is conducted at relatively high
temperatures. To minimize the development of surface defects during such
thermal processing, silicon wafers are surrounded during annealing by a
process atmosphere that does not react with solid silicon.

[0005]Nonetheless, the presence of even a minute quantity of an impurity
in the process atmosphere that surrounds a silicon wafer during annealing
can cause pitting to occur in the surface of the silicon wafer. The
presence of pits in the surface of a silicon wafer will reduce the
reliability of semiconductor devices manufactured from that wafer.
Defects in the surface of a silicon wafer can, for example, dramatically
degrade the integrity of any gate oxide subsequently formed over that
surface defect.

SUMMARY OF THE INVENTION

[0006]Accordingly, the present invention provides apparatus and methods
that minimize the development of surface defects in a silicon wafer
during thermal processing, particularly during thermal processing at the
relatively high temperatures at which silicon wafers are annealed. Such
relatively high temperatures range above and below a typical annealing
temperature of about 1200 degrees Centigrade. Nonetheless, the principles
and concepts residing among the teachings of the present invention will
in addition have applicability to the thermal processing of silicon
wafers in controlled conditions at less extreme temperature, and for
purposes other than for annealing.

[0007]The present invention has utility relative, both to
horizontally-disposed furnaces for silicon wafers, as well as to
vertically-oriented furnaces in which wafers of six or more inches in
diameter are thermally processed.

[0008]Certain features and advantages of the invention have been generally
described in this summary section; however, additional features,
advantages, and embodiments are presented herein or will be apparent in
view of the drawings, specification, and claims hereof. Accordingly, it
should be understood that the scope of the invention is not to be limited
by the particular characterizations presented in this summary section.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]Reference will be made to embodiments of the present invention,
examples of which are shown in the accompanying figures. Those figures
are intended to be illustrative, however, rather than limiting. Although
the present invention is generally described in the context of the
embodiments shown in the accompanying figures, the scope of the present
invention is not to be restricted to the details of those particular
embodiments.

[0010]FIG. 1 is a diagram of an embodiment of an apparatus for thermally
processing silicon wafers that incorporates teachings of the present
invention.

[0011]FIG. 2 is a flow chart of an embodiment of a method for thermally
processing silicon wafers that incorporates teachings of the present
invention.

[0012]FIG. 3 is a flow chart of one embodiment of a subroutine for
performing the step of purging the staging area in the method of FIG. 2.

[0013]FIG. 4 is a flow chart of one embodiment of a subroutine for
performing the step of transferring silicon wafers in the method of FIG.
2.

[0014]FIG. 5 is a flow chart of one embodiment of a subroutine for
performing the step of surrounding silicon wafers in the method of FIG.
2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015]In the following description, for the purpose of explanation,
specific details are set forth in order, by the use of concrete examples,
to provide a sound understanding of the present invention. It will be
apparent, however, that the present invention may be practiced without
some or even all of those details, and it will be recognized that
embodiments of the present invention, some of which are described below,
may be incorporated into a number of different device, systems, and
methods. Structures, device, and methods depicted in block diagram are
merely illustrative of exemplary embodiments of the present invention and
are included in that form in order to avoid obscuring essential teachings
of the present invention.

[0016]Furthermore, connections between components or between method steps
in the figures are not restricted to connections that are effected
directly. Instead, connections illustrated in the figures between
components or method steps may be modified or otherwise changed through
the addition thereto of intermediary components or method steps, without
departing from the teachings of the present invention.

[0017]Reference in the specification to "one embodiment" or "an
embodiment" indicate that a particular feature, structure,
characteristic, function, or method step described in connection with
that embodiment is included in at least one embodiment of the present
invention. The various uses of the phrase "in one embodiment" at
different locations throughout the specification do not necessarily
constitute multiple references to a single embodiment of the present
invention.

[0018]A. Apparatus for Thermally Processing Silicon Wafers

[0019]FIG. 1 depicts elements of one embodiment of an apparatus 10 that
incorporates teachings of the present invention. Apparatus 10 is,
thereby, capable of undertaking the thermal processing of a silicon wafer
and minimizing the development of defects in the surface of the wafer
during that processing. Apparatus 10 is so sized as to be able to
function in this manner relative to one or to a plurality of silicon
wafers. For convenience and consistency in the accompanying figures and
in the detailed discussion that follows, a plurality of silicon wafers
will be depicted and addressed. Doing so is not intended, however, to
preclude the use of apparatus 10 to thermally process solely a single
silicon wafer.

[0020]Central to apparatus 10 is a selectively-sealable process tube 12
that defines therewithin a heat treatment area for silicon wafers.
Process tube 12 is so configured as to be able to receive silicon wafers
into that heat treatment area, and after being sealed, enclose the
silicon wafers during warming of process tube 12 and the wafers therein
to a predetermined temperature T. When silicon wafers are to be annealed
in process tube 12, predetermined temperature T will be in a range of
from about 1175 degrees Centigrade to about 1250 degrees Centigrade. More
typically, annealing is conducted at a predetermined temperature T of
about 1200 degrees Centigrade. Nonetheless, apparatus and systems that
thermally process silicon wafers at other temperatures, or toward other
objectives, will also benefit from teaching of the present invention, and
particularly form the inclusion of all or some of the exemplary elements
and subsystems included in apparatus 10 shown in FIG. 1.

[0021]Process tube 12 is an elongated tubular structure, possibly but not
necessarily of cylindrical construction, that has a closed end 16 and an
open end 18 that includes an opening 20 into the heating area within
process tube 12. As depicted, by way of example and not by way of
limitation, the longitudinal axis of process tube 12 is oriented
vertically, whereby closed end 16 of process tube 12 is positioned at the
top of process tube 12, and open end 18 of process tube 12 is positioned
at the bottom of process tube 12. Such an orientation in a process tube
is the hallmark of a vertically-oriented furnace for silicon wafers.
Accordingly, it is likely, but not necessary, that process tube 12 is
possessed of a relatively large inner diameter of ten or more inches by
which to encompass for thermal processing silicon wafers of six or more
inches in diameter. The inner diameter of process tube 12 may, however,
be of a smaller dimension. Whether or not that is the case, the
longitudinal axis of process tube 12 may be oriented horizontally, in
which case the apparatus incorporating process tube 12 would be a
horizontally-oriented furnace for silicon wafers. Typically, process tube
12 is comprised of a material, such as quartz, that is structurally
stable at high temperatures.

[0022]By way of illustration, process tube 12 of apparatus 10 is shown
enclosing a plurality of silicon wafers 22 that are to be heated to
predetermined temperature T and maintained at that temperature for a
corresponding treatment period. Typically, silicon wafers that are to be
subjected to thermal processing, such as to annealing, are supported from
a structure that is sufficiently mobile among the elements of a thermal
processing device, such as apparatus 10, as to be able to move into and
out of process tube 12, ferrying the wafers to and from the thermal
processing step that is to be conducted therein. Accordingly, such a
mobile support structure for silicon wafers is referred to in the
relevant industry as "a boat", and for convenience of illustration a
nonrepresentational version of a silicon wafer transport boat 24 is shown
positioned in process tube 12 supporting seven silicon wafers 22.
Transport boat 24 could as well support merely a single silicon wafer 22.

[0023]The placement of silicon wafers 14 onto transport boat 24 and the
removal of silicon wafers from transport boat 24 occur, not in process
tube 12, but rather in another portion of apparatus 10, a load lock 26
that is positioned at open end 18 of process tube 12. The interior of
load lock 26 functions as a staging area in which silicon wafers are
assembled onto and unloaded from a transport boat. Lock 26 has a closed
end 28 and an open end 30. As the arrangement of components shown by way
of example in FIG. 1 is an exemplary vertically-oriented furnace, load
lock 26 is positioned below process tube 12, closed end 28 of load lock
26 is at the top of load lock 26, and open end 30 of load lock 26 is at
the bottom. Open end 30 of load lock 26 includes an opening 32 into the
staging area within load lock 26, and open end 30 of load lock 26 is
located directly opposite, or below, opening 20 into process tube 12. In
some thermal processing devices, a load lock, such as load lock 26 in
apparatus 10, is provided with a degree of mobility that permits the load
lock to travel at least toward and away from an associated process tube,
such as process tube 12.

[0024]For illustrative purposes, load lock 26 is shown to enclose the same
transport boat and the same seven silicon wafers as are disposed in
process tube 12. In the interest of for clarity, however, when positioned
in load lock 26 those structures will be depicted in dashed lines and
identified, respectively, as a transport boat 24A and as silicon wafers
22A.

[0025]Interposed between opening 20 of process tube 12 and opening 32 of
load lock 26 is a planar closure cap 34. Closure cap 34 is capable
selectively of closing opening 20 at open end 18 of process tube 12.
Closure cap 34 is advanced in a direction suggested by arrow A toward
process tube 12. This closes the gap G between process tube 12 and cap
34, eventually allowing closure cap 34 to sealingly engage the sides of
open end 20 of process tube 12 at the periphery of opening 20. Process
tube 12 is sealed in this manner only after transport boat 24 has entered
process tube 12 carrying silicon wafers 22, but before the commencement
of the step of heating silicon wafers 22 in process tube 12. Following
the step of heating silicon wafers 22 in process tube 12, closure cap 34
is moved away from process tube 12 in a direction suggested by arrow B to
unseal opening 20 into process tube 12. Transport boat 24 and silicon
wafers 22 are then extracted from process tube 12, and silicon wafers 22A
are unloaded from transport boat 24A in load lock 26.

[0026]To sustain the purity of the atmosphere surrounding transport boat
24 and silicon wafers 22 in process tube 12 once silicon wafers 22 are
eventually subjected to thermal processing, apparatus 10 includes a cap
purge system 36. The elements of cap purge system 36 are so constituted
and configured as to deliver argon gas Ar across gap G, when closure cap
36 is being advanced into sealing engagement with opening 20.
Accordingly, cap purge system 36 includes a pressurized argon gas source
38, a network of argon gas purge channels 40 formed within cap 34, and an
argon gas delivery line 42 that communicates argon gas under positive
pressure from argon gas source 38 to argon gas purge channels 40 in
closure cap 34. Argon gas purge channels 40 are so configured as to expel
argon gas Ar from closure cap 34 toward process tube 12 immediately
inside the entire periphery of opening 20 thereinto.

[0027]Argon is a gas that is non-reactive with solid silicon at all
temperatures relevant to the thermal processing of silicon wafers, and
particularly at any predetermined temperature T at which annealing can be
effected. Cap purge system 36 insures that any gas joining the atmosphere
in the heating area inside process tube 12 during the closure of gap G by
the movement of cap 34 toward opening 20 is a gas that, like argon, is
inert to solid silicon at predetermined temperature T.

[0028]To preclude any compromise of the purity of argon gas Ar supplied by
cap purge system 36 into process tube 12, the flow of argon gas Ar
through argon gas delivery line 42 is controlled by a pneumatic control
valve 44 that is argon-activated. In this manner, should routine measures
be unsuccessful in segregating pneumatic control gases from the valved
gas, any leakage of pneumatic control gas into argon gas Ar of cap purge
system 36 will be simply more argon, a gas that is non-reactive with
solid silicon at predetermined temperature T.

[0029]In another aspect of the present invention, apparatus 10 includes a
load lock purge system 46 that communicates with load lock 26 and induces
therethrough a positive flow of a staging gas X chosen from a group of
gasses comprising hydrogen (H2) and argon (Ar). Load lock purge
system 46 is operated during the time that silicon wafers 22A are being
assembled onto transport boat 24A in load lock 26 preparation to be moved
into process tube 12. Both hydrogen and argon are non-reactive with solid
silicon at any predetermined temperature T. Consequently, replacing the
atmosphere surrounding silicon wafers 22A and transport boat 24A during
assembly in load lock 26 insures that gases that might react with the
wafers during thermal processing do not linger about or attach to those
structures once silicon wafers 22A and transport boat 24A are eventually
moved into process tube 12 for thermal processing.

[0030]Load lock purge system 46 includes a pressurized staging gas source
48, a staging gas delivery line 50 that communicates staging gas X to
load lock 26, and a load lock outlet line 52 that vents gas from load
lock 26 during operation of load lock purge system 46. A load lock
impurity sensor 54 is located in load lock outlet line 52 to monitor the
quality of gas being vented from load lock 26 during operation of load
lock purge system 46. In particular, load lock impurity sensor 54 is
intended to monitor the gas vented from load lock 26 for moisture
(H2O) and oxygen (O2). Load lock impurity sensor 54 generates
an electrical or other signal S54 reflective of the content of the
gas vented from load lock 26. When signal S54 reflects that the
concentration of moisture and the concentration of oxygen in the gas
being vented from load lock 26 are below respective predetermined
allowable levels, system 10 proceeds to move silicon wafers 22A and
transport boat 24A out of load lock 26 and into process tube 12. The
variety of signal S54 reflecting that the concentration of moisture
and the concentration of oxygen in the gas being vented from load lock 26
are below respective predetermined allowable levels thus functions as a
go-condition signal for further processing of silicon wafers 22A, while a
contrary form of signal S54 functions as a stop-condition signal for
system 10.

[0031]To preclude any compromise of the purity of staging gas X supplied
by cap purge system 46 into load lock 26, the flow of staging gas X
through staging gas delivery line 50 is controlled by a pneumatic control
valve 56 that is argon-activated. In this manner, should routine measures
be unsuccessful in segregating pneumatic control gases from the valved
gas, any leakage of pneumatic control gas into staging gas X of load lock
purge system 46 will be simply argon, a gas that is non-reactive with
solid silicon at predetermined temperature T.

[0032]In another aspect of the present invention, apparatus 10 includes a
heating atmosphere supply system 58 that communicates with load lock 12
and induces therethrough a positive flow of a process gas Y that is
non-reactive with solid silicon at predetermined temperature T. Process
gas Y is chosen from a group of gasses comprising hydrogen (H2) and
argon (Ar). Heating atmosphere supply system 58 is operated during the
time that silicon wafers 22 are being heated in process tube 12 to
predetermined temperature T. Both hydrogen and argon are non-reactive
with solid silicon at any predetermined temperature T. Consequently,
replacing the atmosphere surrounding silicon wafers 22 during thermal
processing insures that gases that might react with the wafers then are
not present to do so.

[0033]Heating atmosphere supply system 58 includes a pressurized process
gas source 60, a process gas supply line 62 that communicates process gas
Y to process tube 12, and a point-of-use purifier 64 interposed in
process gas supply line 62 to the flow of process gas Y therethrough.
Point-of-use purifier 64 includes a filter 66 of particulate media in
process gas Y and an absorbent 68 of metallic contaminants in process gas
Y. Particulate media and metallic contaminants in the atmosphere
surrounding silicon wafers 22 during thermal processing would contribute
to the development defects in silicon wafers 22. As a result, regardless
of the purity of process gas Y in pressurized process gas source 60,
immediately prior to actual use in process tube 12 process gas Y is
relieved by point-of-use purifier 64 of any latent or acquired
contaminants that could give rise to surface defects in silicon wafers 22
during thermal processing.

[0034]Also contributing to the purity of process gas Y supplied to process
tube 12, and thus part of heating atmosphere supply system 58, is a
bypass loop 70 that communicates with process gas supply line 62 and
draws therefrom a sample Z of the gas that is flowing through process gas
supply line 62 toward process tube 12. Located in process gas supply line
62 is a dew point meter 72 that detects moisture (H2O) in sample Z
of the gas being drawn through bypass loop 70. Moisture in the atmosphere
surrounding silicon wafers 22 would react therewith at predetermined
temperature T giving rise to defects in silicon wafers 22 during thermal
processing.

[0035]Dew point meter 72 generates an electrical or other signal S72
that is reflective of the content of sample Z of gas drawn through bypass
loop 70. When signal S72 reflects that the concentration of moisture
in sample Z of the gas being drawn through bypass loop 70 is below a
predetermined allowable level, system 10 allows the thermal processing of
silicon wafers 22 to commence. Thermal processing continues in process
tube 12 so long as this prerequisite condition is maintained in the gas
being drawn through bypass loop 70. The variety of signal S72
reflecting that the concentration of moisture in sample Z of the gas
being drawn through bypass loop 70 is below a predetermined allowable
level thus functions as a go-condition signal for thermal processing of
silicon wafers 22, while a contrary form of signal S72 functions as
a stop-condition signal for system 10 in that regard.

[0036]To preclude any compromise of the purity of process gas Y supplied
by heating atmosphere supply system 58 to process tube 12, the flow of
process gas Y through process gas supply line 62 is controlled by a
pneumatic control valve 74 that is argon-activated. In this manner,
should routine measures be unsuccessful in segregating pneumatic control
gases from the valved gas, any leakage of pneumatic control gas into
process gas Y of heating atmosphere supply system 58 will be simply
argon, a gas that is non-reactive with solid silicon at predetermined
temperature T.

[0037]Functioning in harmony with heating atmosphere supply system 58,
apparatus 10 is provided with a process tube outlet line 76 that vents
gas from process tube 12 during operation of heating atmosphere supply
system 58. A process tube impurity sensor 78 is located in process tube
outlet line 76 to monitor the quality of gas being vented from process
tube outlet line 76 during operation of heating atmosphere supply system
58. In particular, process tube impurity sensor 78 is intended to monitor
the gas vented from process tube 12 for the presence therein of moisture
(H2O) and oxygen (O2). Process tube impurity sensor 78
generates an electrical or other signal S78 that is reflective of
the content of the gas vented from process tube 12. When signal S78
reflects that the concentration of moisture and the concentration of
oxygen in the gas being vented from process tube 12 are each below
respective predetermined allowable levels, system 10 proceeds to
thermally process silicon wafers 22 in process tube 12. Thermal
processing continues in process tube 12 so long as this prerequisite
condition is maintained in the gas being vented from process tube 12. The
variety of signal S78 reflecting that the concentration of moisture
and the concentration of oxygen in the gas being vented from process tube
12 are each below respective predetermined allowable levels, thus
functions as a go-condition signal for further processing of silicon
wafers 22, while a contrary form of signal S78 functions as a
stop-condition signal for system 10.

[0038]B. Method for Thermally Processing Silicon Wafers

[0039]The present invention also contemplates a method for thermally
processing silicon wafers in order to minimize the formation of defects
in the surface of those wafers during the processing.

[0040]FIG. 2 is a flow chart that provides an overview of an embodiment of
a method 100 incorporating teachings of the present invention for
thermally processing silicon wafers. Method 100 includes the step
identified in process box 102 of assembling in a staging area a silicon
wafer that is to be thermally processed and the step identified in
process box 104 of purging the atmosphere in the staging area with a
staging gas that is non-reactive with solid silicon at a predetermined
treatment temperature T. The staging gas is selected from a group of
gasses comprising hydrogen (H2) and argon (Ar). Method 100 also
includes the step identified in process box 106 of transferring the
silicon wafer from the staging area to a heat treatment area, the step
identified in process box 108 of surrounding the silicon wafer in the
heat treatment area with the process gas, and the step identified in
process box 110 of heating the silicon wafer to predetermined temperature
T in the process gas in the heat treatment area.

[0042]FIG. 3 is a flow chart of one embodiment of a subroutine for
performing the step of purging the staging area identified in process box
104 in method 100 of FIG. 2. The step of purging the staging area
identified in process box 104 follows or is conducted simultaneously with
the step identified in process box 102 of assembling a silicon wafer in a
staging area, but both the step identified in process box 102 of
assembling a silicon wafer and the illustrated subroutine for performing
the step of purging the staging area identified in process box 104
precede the step identified in process box 106 of transferring the
silicon wafer from the staging area to a heat treatment area.

[0043]The illustrated subroutine for performing the step of purging the
staging area includes the step of inducing a positive flow of the staging
gas through the staging area identified in process box 112, and the step
of venting gas from the staging area during that positive flow of the
staging gas identified in process box 114. Decision diamond 116 then
requires an evaluation of whether the vented gas from the staging area
contains unacceptably high concentrations of oxygen or of moisture. Doing
so involves the steps of monitoring gas vented from the staging area to
detect the presence therein of each of oxygen and of moisture, and
proceeding with the step of transferring the silicon wafer identified in
process box 106 only when the concentration of oxygen and the
concentration of moisture detected in the gas vented from the staging
area are below respective predetermined allowable levels. If a
concentration of oxygen or a concentrating of moisture is detected in the
gas vented from the staging area that is not below those respective
predetermined allowable levels, then the subroutine for performing the
step of purging the staging area is continued, until concentrations of
such contaminants are below those respective predetermined allowable
levels.

[0044]FIG. 4 is a flow chart of one embodiment of a subroutine for
performing the step of transferring silicon wafers identified in process
box 106 in method 100 of FIG. 2. The step of transferring silicon wafers
identified in process box 106 follows the step identified in process box
104 of purging the atmosphere in the staging area with a staging gas, but
precedes step identified in process box 108 of surrounding the silicon
wafer in the heat treatment area with the process gas.

[0045]The illustrated subroutine for performing the step of transferring
silicon wafers includes the step of removing a barrier interposed between
the staging area and an opening into the heat treatment area identified
in process box 118, the step of moving the silicon wafer from the staging
area through the opening into the heat treatment area identified in
process box 120, and the step identified in dashed process box 122 of
using the barrier to close the opening into heat treatment area. That
step of closing the opening into heat treatment area with the barrier
itself includes the step of advancing the barrier into sealing engagement
with the sides of the opening into the heat treatment area identified in
process box 124 and the step identified in process box 126 of delivering
argon gas across a gap between the barrier and the sides of the opening
into the heat treatment area during the step of advancing the barrier.

[0046]FIG. 5 is a flow chart of one embodiment of a subroutine for
performing the step of surrounding silicon wafers identified in process
box 108 in method 10 of FIG. 2. The step of surrounding silicon wafers
identified in process box 108 follows the step identified in process box
106 of transferring the silicon wafer from the staging area to a heat
treatment area, but precedes and is conducted simultaneously with the
step identified in process box 110 of heating the silicon wafer to
predetermined temperature T.

[0047]The illustrated subroutine for performing the step of surrounding
silicon wafers includes the step identified in dashed process box 128 of
inducing through the heat treatment area a positive flow of the process
gas, evaluating whether the vented gas from the staging area contains
unacceptably high concentrations of oxygen or of moisture as called for
in decision diamond 136, and proceeding, albeit through other steps in
the illustrated subroutine, toward the step of heating the silicon wafer
to predetermined temperature T identified in process box 110 only when
the vented gas from the staging area contains concentrations of oxygen
and concentrations of moisture that are below respective predetermined
allowable levels. If the vented gas from the staging area does contains
concentrations of hydrogen or concentrations of moisture that are not
below those respective predetermined allowable levels, method 100
continues the step of inducing through the heat treatment area a positive
flow of the process gas identified in dashed process box 128.

[0048]The step identified in dashed process box 128 of inducing through
the heat treatment area a positive flow of the process gas includes the
step of purifying the process gas to a predetermined quality level
identified in process box 130, the step identified in process box 132 of
removing from the pure process gas all particulate media and all metallic
contaminants, and the step identified in process box 134 of supplying the
pure process gas to the heat treatment area, which should occur after the
step of removing media and metallic contaminants from the pure process
gas.

[0049]The subroutine illustrated in FIG. 5 for performing the step of
surrounding silicon wafers includes the step identified in process box
138 of venting gas from the heat treatment area during positive flow of
the process gas therethrough, and as indicated in decision diamond 140
the step of monitoring gas vented from the heat treatment area to detect
the presence therein of unacceptably high concentrations of each of
hydrogen and of moisture. Only when the concentration of oxygen and the
concentration of moisture detected in the gas vented from the heat
treatment area are below respective predetermined allowable levels, does
method 100 proceed with the step of heating the silicon wafer to
predetermined temperature T identified in process box 110. Should
concentrations of hydrogen or concentrations of moisture be detected in
the gas vented from the heat treatment area that are not below those
respective predetermined allowable levels, method 100 is terminated, and
the source of those contaminates is investigated and eliminated.

[0050]The foregoing description of the invention has been described for
purposes of clarity and understanding. It is not intended to limit the
invention to the precise form disclosed. Various modifications may be
possible within the scope and equivalence of the appended claims.